Polyphenylene ether resins are attracting attention as being useful by their excellent mechanical properties and heat resistance and have been used in polymer blends with, for example, a styrene resin. However, since the polyphenylene ether resin is seriously inferior in solvent resistance, it has been proposed to eliminate this disadvantage by blending with polyamide as described in U.S. Pat. No. 4,338,421 or with polyester as described in Japanese Patent Publication No. 21662/76. Aiming at further improvements on impact strength of these polymer blends, it has also been proposed to incorporate, into the combination of polyphenylene ether and polyamide, a compound containing a carboxyl group, an imide group, an epoxy group, etc. as disclosed in U.S. Pat. No. 4,315,086, or to further incorporate a rubbery substance into such a resin composition as disclosed in Japanese Laid-Open Patent Application No. 49753/81.
On the other hand, plastic materials have recently been demanded to satisfy performance requirements of higher levels than ever before with a broadening of application of them. For example, the above-described polymer blends, though excellent in heat resistance and organic solvent resistance, have been demanded to have further enhanced mechanical strength. In order to meet the demand, the inventors previously developed a composition comprising a polyphenylene ether resin, a polyamide resin, and a compound containing an unsaturated group and a polar group per molecule, having incorporated therein an alkenyl aromatic compound/conjugated diene block copolymer having a copolymerization ratio within a specific range as disclosed in Japanese Laid-Open Patent Application No. 127339/87. When applied to the field of large-sized and plate-shaped industrial parts, however, this composition showed a tendency to be as yet not quite satisfactory in planar impact strength that is of importance in such application, leaving room for further improvements.